American tennis player Christian Harrison has been fined $3,000 ($A4,000) by the ATP after refusing to take part in a mandatory on-court interview at the Delray Beach Open in Florida that required him to wear a mask.

The qualifier, ranked 789th in the world, stunned top seed Cristian Garin of Chile 7-6 (7-3) 6-2 on Saturday for his first victory against a top-50 player.

He decided not to wear a mask following his win and declined to be interviewed on court.

The 26-year-old spoke out against wearing masks on social media last week saying: “Healthy enough to play 3-hour matches. But I’m an absolute safety hazard walking maskless through a restaurant.”

“Christian Harrison was fined $3,000 for declining the mandatory post-match on-court interview following his second-round win over Cristian Garin on Saturday,” the ATP said in a statement.

Harrison reached his maiden ATP semi-final by defeating Gianluca Mager 7-6 (7-2) 6-4 at the event on Monday.

He wore a mask for the subsequent on-court interview and later said in an Instagram post he was not aware it was mandatory to wear one for TV interviews.

He added the ATP supervisor had told him the fine would be rescinded if he appealed and promised to follow tournament rules even if he did not agree with them.

“It’s not healthy to wear a mask in the hot sun for more time than absolutely necessary after a tough match. I was given the impression it was optional. I wasn’t told I would be fined,” he said in the post.

“I was required to do a sit-down interview after the match with the same person which I agreed to and I did not have to wear a mask for it. It’s not about safety to wear mask for TV to talk to a microphone with no one around.

“I don’t support masking people for the sake of it for TV.”

Harrison, who has required surgery eight times in his career, has just five ATP Tour match victories with three of them coming in the past week.

The 26-year-old will meet fourth-seeded Hubert Hurkacz for a place in the final.

Hurkacz, the 23-year-old world No. 35, will be heavily favoured to beat Harrison as he chases just a second ATP Tour trophy.